1. Field of the Invention
The invention relates to a process for the continuous production of aryl carbonates from carbonates containing at least one aliphatic ester group and phenols, on the one hand, and from alkyl aryl carbonates, on the other hand, by catalysed transesterification, in which process the reaction is carried out in at least two stirred containers connected one behind the other.
The production of aromatic and aliphatic-aromatic carbonic acid esters (carbonates) by transesterification starting from aliphatic carbonic acid esters and phenols is known in principle. It is an equilibrium reaction in which the position of the equilibrium is displaced almost completely in the direction of the aliphatically substituted carbonates. It is therefore comparatively easy to produce aliphatic carbonates from aromatic carbonates and alcohols. In order to carry out the reaction in the reverse sense in the direction of aromatic carbonates, it is necessary to displace the very unfavourably situated equilibrium efficiently, not only a very active catalyst compound but also a favourable procedure having to be used.
2. Description of the Related Art
A multiplicity of efficient catalysts, such as, for example, alkali metal hydroxides, Lewis acid catalysts from the group comprising the metal halides (German Offenlegungsschrift 2 528 412 and 2 552 907), organotin compounds (European Patent Specification 0 000 879, European Patent Specification 0 000 880, German Offenlegunsschrift 3 445 552, European Patent Specification 0 338 760), lead compounds (Japanese Patent Specification 57/176 932) and Lewis acid/protonic acid catalysts (German Offenlegungsschrift 3 445 553) are recommended for the transesterification of aliphatic carbonic acid esters with phenols.
In the known processes, the transesterification is carried out in a reactor which is operated batchwise at atmospheric pressure or under pressure, if necessary with an additional separating column. In these processes, reaction times of many hours are needed before even only moderate conversions of approximately 50% phenol are achieved even with the most active catalysts. Thus, in the transesterification of phenol with diethyl carbonate at 180.degree. C. with batchwise operation using various organotin compounds such as those described in German Offenlegungsschrift 3 445 552, yields of diphenyl carbonate in an order of magnitude of more than 20% are achieved only after an approximately 24-hour reaction time; in the transesterification of phenol and dimethyl carbonate with batchwise operation with the aid of organotin catalysts such as those described in European Patent Specification 0 000 879, the phenol conversion is 34% of the theoretical value after 30 h.
This means that, owing to the unfavourable thermodynamic conditions, the transesterification reactions described with batchwise operation can be carried out only very disadvantageously for the purpose of an industrial process even if very active catalyst systems are used since very poor space-time yields and high dwell times at high reaction temperatures are necessary.
Such procedures are also particularly disadvantageous since, even with very selective transesterification catalysts, an appreciable proportion of side reactions, for example the formation of ethers and the cleaving of carbon dioxide, occur at the high temperatures and with long dwell times of many hours.
An attempt has therefore been made to displace the reaction equilibrium as quickly as possible in the direction of the products by adsorption of the alcohol produced during the transesterification on molecular sieves (German Offenlegungsschrift 3 308 921). The description of this procedure reveals that the adsorption of the reaction alcohol requires a large amount of molecular sieve which exceeds by far the amount of the alcohols liberated. Furthermore, the molecular sieves used have to be regenerated even after a short time and the rate of conversion to the alkyl aryl carbonate intermediates is relatively low. This process, too, therefore appears not to be advantageously applicable industrially and economically.
A continuous transesterification process for the production of aromatic carbonates in which the reaction is carried out in one or more multistage distillation columns is claimed in EP-A 0 461 274. In this case, phenols are first reacted with dialkyl carbonates to form aryl carbonate mixtures which essentially contain alkyl aryl carbonates. These are then further reacted to form the desired diaryl carbonate end products in a second, downstream column. The applicant stresses the efficiency and the selectivity of his procedure. This is contradicted by the relatively low space-time yields, specified in the examples, of the reaction of phenols with diaryl carbonates, these having been achieved under optimum conditions at high temperatures and pressures with the best transesterification catalysts. In the specified procedure which emerges from the examples, the further reaction of the alkyl acryl carbonates to form diaryl carbonates proceeds as a disproportionation reaction. It is therefore not surprising that substantially better space-time yields are achieved in this reaction which proceeds rapidly compared with the first transesterification stage.
The object of an improvement of the transesterification reaction should therefore be a further acceleration, especially of the transesterification stages with phenol, but the selectivity of the entire process should not be reduced.
Surprisingly, it has now been found that this is achieved in a continuously run transesterification process in stirred-kettle cascades, although the particular efficiency and the mild conditions of the column procedure in contrast to the kettle procedure are stressed and given prominence in EP-A 0 461 274.
Markedly higher space-time yields of the alkyl aryl carbonate formation than disclosed in EP-A 0 461 274 can be achieved with the continuous procedure according to the invention in stirred kettles connected one behind the other even at normal pressure and substantially lower temperatures. In view of the arguments put forward in EP-A 0 461 274 (page 5, line 39 ff.), however, the fact that these higher conversion rates are achieved with very high selectivity of the reactions of &gt;99% is quite particularly surprising. The procedure according to the invention therefore must also be assessed as particularly advantageous because very simple and easily controllable technology using standard equipment is used. The design of such equipment and the extrapolation of a continuously operated stirred-kettle process to the industrial scale is relatively easy for the person skilled in the art to carry out. Temperature, pressure and dwell time spectrum of the reactants can easily be adjusted over a wide range, with the result that a variable procedure is also available. The heat input necessary for the endothermically proceeding transesterification reaction can be achieved here without problems.